Xcode 4.2的编译改动(ARC)

关于ARC

ARC（Automatic Reference Counting）是一种据说可以自动释放内存的方式，但带来了不少弊端，例如：

·如果结构复杂一些的view，系统可能会释放错误。

·由于内存是自动释放，所以retain/release/autorelease/dealloc也不能使用了（也不需要使用了），如果使用，反而会报错，如

ARC forbids explicit message send of release

或者

'release' is unavailable: not available in automatic reference counting mode..

显然ARC和release是不能共存的，想要编译通过就要干掉一个。

通常被干掉的会是ARC：

在老版的xcode中，打开“Build Setting”,找到“Objective-C Automatic Reference Counting”项，将它的值设置成“NO”

在较新的Xcode中如4.2，打开“Build Setting”，找到"CLANG_ENABLE_OBJC_ARC" 将它的值设置成NO

关于strong和weak

强引用与弱引用的广义区别：

强引用strong也就是我们通常所讲的引用，其存亡直接决定了所指对象的存亡。如果不存在指向一个对象的引用，并且此对象不再显示列表中，则此对象会被从内存中释放。

弱引用weak除了不决定对象的存亡外，其他与强引用相同。即使一个对象被持有无数个若引用，只要没有强引用指向他，那麽其还是会被清除。没办法，还是 “强哥” 有面子。

 

简单地讲，

在@property那里用strong代替retain，用weak代替assign。

weak比assign多了一个功能，当对象消失后自动把指针变成nil，好处不言而喻。 

关于@Autoreleasepool

NSAutoReleasePool  被 @ {Autoreleasepool / / Code for autoreleasepool } block 取代了。所起的作用还是一致的。 

附上官方原文：

Automatic Reference Counting

Automatic Reference Counting (ARC) is a compiler-level feature that simplifies the process of managing the lifetimes of Objective-C objects. Instead of you having to remember when to retain or release an object, ARC evaluates the lifetime requirements of your objects and automatically inserts the appropriate method calls at compile time. 

To be able to deliver these features, ARC imposes some restricti*****—primarily enforcing some best practices and disallowing some other practices:

Do not call the retain, release, autorelease, or dealloc methods in your code.

In addition, you cannot implement custom retain or release methods. 

Because you do not call the release method, there is often no need to implement a custom dealloc method—the compiler synthesizes all that is required to relinquish ownership of instance variables. You can provide a custom implementation of dealloc if you need to manage other resources.

Do not store object pointers in C structures.

Store object pointers in other objects instead of in structures.

Do not directly cast between object and nonobject types (for example, between id and void*).

You must use special functi***** or casts that tell the compiler about an object’s lifetime. You use these to cast between Objective-C objects and Core Foundation objects.

You cannot use NSAutoreleasePool objects.

Instead, you must use a new @autoreleasepool keyword to mark the start of an autorelease block. The contents of the block are enclosed by curly braces, as shown in the following example 

@autoreleasepool

{

  // Your code here

}

ARC encourages you to think in terms of object graphs, and the relati*****hips between objects, rather than in terms of retain and release. For this reason, ARC introduces new lifetime qualifiers for objects, including zeroing weak references. The value of a zeroing weak reference is automatically set to nil if the object to which it points is deallocated. There are qualifiers for variables, and new weak and strong declared property attributes, as illustrated in the following examples:

// The following declaration is a synonym for: @property(retain) MyClass *myObject;

@property(strong) MyClass *myObject;

// The following declaration is similar to "@property(assign) MyOtherClass *delegate;"

// except that if the MyOtherClass instance is deallocated,

// the property value is set to nil instead of remaining as a dangling pointer

@property(weak) MyOtherClass *delegate;

Xcode provides migration tools to help convert existing projects to use ARC. For more information about how to perform this migration, see Xcode New Features User Guide. For more information about ARC itself, see Programming With ARC Release Notes.